The present invention is directed to the field of hydraulic pumps, particularly positive displacement pumps such as axial piston pumps and vane pumps. Hydraulic systems are widely used in many power and motion control applications and offer numerous advantages such as high power density, robust performance and relatively low cost. However, hydraulic systems are often noisy. This is often a result of noise produced by the hydraulic pump. Increasingly stringent regulations limiting overall noise in the workplace have increased the need for reducing the noise generated by hydraulic pumps.
A standard axial piston pump and its operation is shown in FIGS. 1A and 1B. A plurality of pistons 10 are provided for receiving hydraulic fluid. The pistons 10 are mounted in a cylinder block 12 which is rotated by a drive shaft 14 and driven by a power source (not shown). As the cylinder block 12 rotates, the pistons 10 are alternately stroked in and out by a yoke 16 which is inclined at a particular angle, typically about 17.5.degree. at full stroke. The pistons are in fluid communication with respective inlet and outlet ports 24, 26 which supply and receive the hydraulic fluid. As the cylinder block 12 is rotated, the piston 10 retracts, expanding the pumping chamber 18. Fluid is drawn in to the pumping chamber 18 from inlet port 24 through valve block 28. The pistons 10 reach their maximum extent at bottom dead center (BDC), after which the pistons 10 extend, collapsing the pumping chamber 18 and thereby discharging the fluid through the valve block 28 into the outlet port 26.
The cylinder block 12 is fluidly connected to the inlet and outlet ports 24, 26 through a valve plate 30 which includes respective inlet and outlet kidney slots 32, 34. The structure and operation of a typical valve plate 30 is shown in FIGS. 2A and 2B. During operation, the rotating pistons 10 draw in hydraulic fluid through the inlet kidney slot 32, the fluid being typically supplied at atmospheric pressure. After the pumping chamber 18 is closed to the inlet 32, it passes BDC, compressing the fluid and discharging the fluid into the outlet kidney slot 34 where it is supplied to the hydraulic system. Such valve plates are advantageous since a variety of valve plates can be interchangeably used to optimize pump operation for a number of different operating conditions.
As the fluid in the pumping chamber is compressed in the transition zone around BDC, the hydraulic fluid reaches a particular chamber pressure (Pc), after which it is discharged through the outlet 34 and into a hydraulic system having a particular system pressure (Ps). However, overpressurization or underpressurization of the piston chamber relative to the hydraulic system has been identified as a source of noise in the hydraulic pump. As seen in FIGS. 3A, an overpressurized piston chamber produces a pressure "overshoot" upon opening to the outlet 34. This overshoot results in a shock which is equivalent to an impact in the system thus producing an audible noise. As seen in FIG. 3B, a very large difference in pressurization produces a large overshoot which results in a louder noise. An underpressurization, as seen in FIG. 3C, also produces noise as the rate of pressure change within the piston chamber is abrupt, and the higher system pressure impacts into the piston chamber. Ideal system operation occurs at conditions where the chamber pressure is equal to system pressure, as shown at FIG. 3D, where the pressure overshoot is zero and the rate of pressure change within the piston chamber is not high.
In order to insure optimally quiet operation, the chamber pressure of the hydraulic pump should be matched to the system pressure. However, several variable factors can affect the pressure profile. Hydraulic pumps can be driven over a wide range of speeds. As the shaft 14 rotates faster, the pistons 10 displace a greater volume of fluid per unit time. Secondly, flow can also be varied by stroke, i.e., the length of piston displacement as determined by the angle of the yoke 16. The yoke 16 can be varied between maximum pitch (to produce maximum piston displacement) and a pitch of zero (to produce zero piston displacement) using the control piston 20 and the bias piston 22. The piston displacement corresponds to the volume of fluid displaced, hence the rate of flow. The third factor that affects pressure within the pumping chamber is hydraulic fluid temperature variation, since it changes the bulk modulus (fluid stiffness) of the fluid.
These variables affect chamber pressurization, thus increasing the noise level during operation when the chamber pressure does not match system pressure as the outlet port opens. However, system pressure may also vary within a hydraulic system over the course of a particular operation. Thus, it is not uncommon for the chamber and system pressures to be unmatched over the majority of variable operational conditions, resulting in generally noisy operation in a standard hydraulic pump.
Noise arises in the pump from deflections in the various components such as the valve block, housing, yoke, and drive shaft. These result from pressure-related forces in the pumping chamber. These deflections are harmonics of the piston pumping frequency. Thus, noise increases in pitch with increased pump speed.
Another source of pump housing vibration is "yoke flutter," an oscillation in the yoke 16 produced by the reciprocating forces of the pistons 10 against the yoke 16. As seen from FIG. 4, each piston 10 applies a moment to the yoke 16 which slightly alters the yoke's pitch and subsequently the stroke of the pistons. Yoke oscillations produce a "pitching" which causes deflections in the pump housing, thereby generating noise. The level of noise is proportional to the magnitude of the change in the yoke moment. The curve 40 shows that for a typical yoke arrangement, the moment can vary by several hundred inch-pounds as a function of chamber angle past bottom dead center (where pumping chamber volume is maximum). The curve 40 also repeats itself over every 360/n degrees of chamber angle, where n is the number of pistons.
Many hydraulic pumps use bushings to support the yoke 16. These bushings tend to have high friction which minimize oscillation of the yoke. Such pumps produce lower levels of noise. However, such bushings are not desirable for pumps which need to make rapid stroke changes. For example, certain injection molding equipment requires a yoke 16 which can vary from zero flow to full flow in several tens of milliseconds. Such a yoke is typically mounted on low-friction roller bearings which permit high speed changes. However, such bearings also permit unwanted displacement variations resulting from yoke moments. The low friction bearings result in a higher level of oscillation, and thus increased levels of noise.
A preferable method of reducing noise is to reduce the alternating forces that produce the deflections in the pump components and the oscillations of the yoke. This is done by using a metering groove 38, as shown in FIGS. 2A and 2B. The metering groove extends into the transition region around BDC and creates a fluid passageway between the piston chamber and the outlet 34. During the standard operation of a hydraulic pump, the piston chamber 18 is "mechanically" pressurized by the forward motion of the pumping piston. As the metering groove 38 meters oil between the chamber and the outlet 34, the pumping chamber is also "hydraulically" pressurized. Thus, the pressure differential between the chamber and the system is equalized, reducing overshoot and the noise produced thereby.
The pressure profile shape is controlled by the shape of the metering groove 38. The design of the grooves is referred to as "pump timing." In addition to overshoots and undershoots as a source of noise, a high rate of pressure change is sufficient to provide a large amount of energy that tends to excite structural resonances, thereby producing noise. Thus, it is also important to control the rates of pressurization so as to control the spectral content of the forcing functions exciting resonances in the pump components. By carefully designing the metering groove 38, the pump timing can be designed to control pressurization so as to produce a minimum rate of pressure change in addition to a minimum overshoot. However, pump timing design can only be "tuned" for a particular pump speed, system pressure and pump stroke. Since these quantities are variables, any low noise pump design must necessarily be a compromise, since a pump must be capable of operating over a wide range of conditions.